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My furnace will run for an hour or two, cycling 3 times each hour, then shuts down. I have to turn the temperature setting to "off" and then back up to 70°F, and it comes back on. I've replaced the thermostat and air filter, cleaned the flame sensor, cleaned the burners, checked all sensors with a meter (all have continuity). It's a Lennox G20Q3E-75-6, fairly old, not sure when it was installed.

Any ideas?

  • Hello, and welcome to Stack Exchange. I've edited your post to use the shift key; please don't yell when you post here. – Daniel Griscom Jan 2 '17 at 12:06
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    Is the thermostat still calling for heat when the furnace shuts down? – ThreePhaseEel Jan 2 '17 at 13:02
  • Does the unit display any error codes? – Tester101 Jan 2 '17 at 15:00
  • I haven't checked to see if thermostat is calling for heat. I'll check....? Am I looking for 24v on the terminal block? – Chris Taylor Jan 2 '17 at 19:56
  • The green light on the ignition control module has not done any flashing. It's usually off until the thermostat turns the ICM on. The green light comes on the the furnace starts up. green light goes out. Runs three cycles in the next hour and then shuts down. I don't have any paper work on the furnace. – Chris Taylor Jan 2 '17 at 20:08
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Some of the electronic ignition modules used in older Lennox furnaces fail by becoming weak or intermittent and after several failed retries on a single start cycle the furnace remains in a no-heat condition until the call for heat is removed and reconnected.

If the failure only occurs during a normal start cycle, and once lit the flame does not go out before the room reaches the thermostat set point, then next time a failure occurs test for 24vac across the power supply leads on the electronic ignition module. If it has power but there is no spark then you probably need a new module.

EDIT:

One word of caution. It would be best to witness the ignition unit failing before ordering a replacement. Rarely a gas valve can intermittently stick closed and cause similar symptoms.

  • The next thing I did was to check the heat pump coils. there was no access panel. so I had to cut open the housing unit to get to the coils. the top of the coils weren't that dirty. the under side, looked like A door mat. Six cans of coil cleaner later. I put it back together. And it ran lick it was supposed to for four hours, Then shut down. I haven't witnessed the failure, for some reason it hasn't failed when I've been there. When it does I will check for voltage. – Chris Taylor Jan 9 '17 at 22:27

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